The present invention relates to the preparation of polyamines. More specifically, the invention relates to a process for preparing methylene diphenylamines by partial neutralization of acidic aniline/aldehyde reaction products.
Generally, it is known in the art to react aniline with formaldehyde in the presence of an acid such as hydrochloric acid to produce methylene diphenylamines. Of particular interest here is the methylene diphenylamines which are useful in the production of polyisocyanates which in turn are useful in the preparation of polyurethane/polyurea foams. Lately, the color of foams has become one of the requirements for judging the quality of foams. Foams which are otherwise acceptable can be rejected because of dark color.
Poor quality of the methylene diphenylamines and consequently the polyisocyanates, at least in part, has been blamed for the discoloration in foams. Hence, a number of improvements in this area have been made in the preparation of higher quality methylene diphenylamines.
U.S. Pat. No. 4,792,624 discloses that polymethylene polyphenyl polyisocyanates of improved color can be obtained from certain polyamines which are prepared by the following process. The process comprises condensing aniline and formaldehyde in the presence of an acid catalyst and is characterized by adding a minor proportion of a polyamine mixture comprising di(aminophenyl)methanes and oligomeric polymethylene polyphenyl polyamines, (collectively known as polymeric MDA) to an intermediate stage of the condensation reaction where the various intermediately formed aminobenzoylamines are present.
U.S. Pat. No. 4,465,639 discloses the production of polymethylene polyphenyl polyamines by addition of controlled amounts of water to the reaction mixture produced by phosgenation of a mixture of polymethylene polyphenyl polyamines (and the like polyamines produced by condensation of formaldehyde and aromatic amines) prior to a complete removal of excess phosgene. The corresponding polymethylene polyphenyl polyisocyanate is said to have significantly improved properties such as color of the polyisocyanates.
U.S. Pat. No. 4,259,526 discloses a process for the preparation of mixtures of polyamines of the polyamino-polyaryl-polymethylene type whereby the proportion of the ortho-substituted products are substantially increased. This increase is brought about by adding the acid catalyst in two stages by reacting an aromatic amine with formaldehyde in the presence of an aqueous acid catalyst, or subjecting a condensate which has been obtained from an aromatic amine and formaldehyde in the absence of a catalyst to a first rearrangement in the presence of an aqueous acid catalyst, followed by adding aqueous acid catalyst to the resulting mixture, and then rearranging the intermediates by heating the mixture to 75 to 150 degrees centigrade. This mixture of polyamines is thereafter recovered.
By the present invention, there is provided another process for preparing polyamines which upon phosgenation produce polyisocyanates of light color.
The present invention encompasses a process for preparing a reaction product containing methylene diphenylamine, comprising reacting aniline and formaldehyde in the presence of an acid catalyst; reducing the acid level of the resulting reaction mixture during digestion of the reaction mixture, followed by completely neutralizing the reaction product after further digestion. Isocyanates corresponding to the polyamines and reaction products thereof, e.g., foams are also encompassed by the present invention.
In the present embodiment of the invention the process for preparing the reaction product containing methylene diphenyl-amine comprises reacting aniline with a formaldehyde in the presence of hydrochloric acid; partially neutralizing the reaction product with sodium hydroxide during the digestion stage of the reaction, followed by completely neutralizing the reaction product after the digestion.
U.S. Pat. No. 4,259,526 which discloses typical,aniline and formaldehyde reactions in the presence of an acid is incorporated herein by reference subject to modifications in accordance with the present invention, as described more fully hereunder.
In accordance with the invention, the reaction product containing methylene diphenylamines of the claimed invention is characterized by low levels of certain impurities. It is believed that increasing levels of these impurities darken the color of polyisocyanates derived from said amines. Generally, methylene diphenylamines can be prepared by reacting aniline with formaldehyde in the ratio of 1.6 to 8.0 and preferably 1.6 to 5.0, in the presence of an acid such as hydrochloric acid.
The amount and type of the acid useful herein is that which can effectively catalyze the subject reaction. The useful acids can be the art-known acids disclosed in the prior art cited above. Illustrative but non-limiting examples of the useful acids include water soluble acids having pKa values below 2.5 and preferably below 1.5. Specific examples include hydrochloric acid which is preferred, hydrobromic acid, sulfuric acid, triflouroacetic acid, methanesulfonic acid, phosphoric acid, formic acid, acetic acid, citric acid and the like. Typically, the acid level is about 10% to 100% and preferably 25% to 50% (molar) based on the aniline. The acid can be added separately or admixed with aniline in a reaction vessel which is usually referred to as xe2x80x9ca mix reactorxe2x80x9d, at the xe2x80x9cinitial stagexe2x80x9d of the reaction. As would be realized, the acid salt of the aniline can be reacted with the formaldehyde
The process of this invention may entail split addition of the reagents. For example, the initial stage of the reaction may be performed in two stages in which portions of the formaldehyde are added in stages.
The reaction, at the initial/condensation stage takes place at a temperature of 0xc2x0 C. to 100xc2x0 C. and preferably 35xc2x0 C. to 45xc2x0 C., at atmospheric pressure and over a period of time sufficient to form intermediates of the reaction product (aminobenzylamines and the like). The period of time can be from about 5 seconds to 60 minutes and preferably 15 minutes to 45 minutes. The reaction can be conducted in one or more reaction vessels, and the progress of the reaction can be monitored by conventional analytical techniques.
At the end of the condensation stage, the reaction mixture is admitted to the digestion stage. By the term xe2x80x9cdigestionxe2x80x9d is meant the stage at which the intermediate aminobenzylamines rearrange to form methylene diphenylamines and polymethylene polyphenylamine. It is a distinct feature of the invention that the process for preparing the methylene diphenylamine comprises the step of reducing the acid level of the reaction mixture during digestion. The acid level is reduced to a level sufficient to effect the rearrangement of the aminobenzylamine over the digestion temperature and residence time described hereunder. The reduction in acid level can be effected by partially neutralizing the reaction mixture to the desired acid level, during the digestion stage. In accordance with the invention, less than the stoichiometric amount of a base required to neutralize the acid is employed. This amount of the base can be from about 10% to 90% and preferably 70 to 80 molar percent based on the amount of acid catalyst employed. The useful base is that which can effectively neutralize the acids described hereinabove. The base can be an art-known one such as disclosed in the prior art cited above. The useful base can be selected from the group consisting of sodium hydroxide, potassium hydroxide and the like. Typically, the digestion stage is conducted at an elevated temperature of about 60xc2x0 C. to 200xc2x0 C. and preferably 90xc2x0 C. to 160xc2x0 C., at a pressure sufficient to control boiling of the reaction mixture, for a period of time (residence time) sufficient to effect the rearrangement of aminobenzylamines into methylene diphenylamines. Typically the residence time is from about 60 minutes to 400 minutes and preferably 100 minutes to 240 minutes. In a multi-vessel, continuous digestion process the partial neutralization is carried out continuously at a point in the process where the reaction mixture has been digested for from 10% to 90% and preferably from 40% to 60% of the total digestion time. After complete digestion, the remainder of the base is added to fully neutralize the acid.
In the practice of the invention, the reaction product containing methylene diphenylamine can be used in the preparation of lighter color isocyanate materials by the reaction of phosgene (phosgenation) with the reaction product containing methylene diphenylamine. The phosgenation reactions can be conducted in the presence of an inert solvent such as chlorobenzene. The polyamines are reacted with phosgene in molar ratios of 1.5 to 20 moles and preferably about 2.2 to 4.0 moles of phosgene per amine group. Upon completion of the phosgenation, the excess phosgene and hydrogen chloride formed are separately or simultaneously removed. The phosgenation product which is present after the removal of excess phosgene and HCl is in the form of a solution and may be separated into a gaseous phase containing volatile compounds having isocyanate groups and a liquid phase which is substantially crude polyisocyanate. The liquid phase can be worked up to produce polyisocyanates in a purer state.
In turn, the lighter color polyisocyanates can be used in the preparation of lighter color isocyanate reaction products such as foams. In the preparation of the foams, the polyisocyanates are reacted with an isocyanate reactive compound such as art-known polyols or polyamines, in the presence of a blowing agent such as carbon dioxide derived from the reaction of the polyisocyanate and water. Art-known auxiliary agents and additives are usually employed in the preparation of the foams